ECOLOGICAL GROUPS 491 



the tall pines which overtop all the other trees receive the 

 sunlight in its full intensity ; under the densest shade of the 

 pines the illumination, compared with that of the unshaded 

 tree tops, may be only one fiftieth or less. 1 



In a forest the number and character of the strata of vege- 

 tation depend largely upon the kinds and density of the trees 

 that compose the uppermost stratum. In an average deciduous 

 forest such as is often found throughout the Central States 

 the uppermost stratum consists of trees such as oaks, maple, 

 beech, elm, hickory, and tulip (Fig. 319). In some regions 

 one finds an almost pure beech or maple forest. The oak- 

 maple or oak-maple-beech combination is not uncommon. If 

 these trees grow closely crowded, the shade underneath is very 

 dense and few shorter trees or shrubs are found. When the 

 forest is more open a second stratum of young trees and tall 

 shrubs is found. This stratum, in addition to the young trees, 

 may contain the bladderwort (Stapliylea), the hop hornbeam 

 (Ostrya), the ironwood (Carpinus), and many others. There 

 may be a stratum of lower shrubs immediately below the tall- 

 shrub stratum, and this lower-shrub stratum often is charac- 

 terized by the blackberry, raspberry, and the greenbrier or cat 

 claw. The next stratum usually is characterized by many tall 

 ferns and many kinds of flowering plants that thrive in deep 

 shade. Below the ferns and upon the soil or decaying vegeta- 

 tion is the lowest stratum, which may include many kinds of 

 toadstools and mushrooms and other fungi, mosses of many 

 kinds, occasionally some of the leafy liverworts, and some of 

 the soil lichens. In some forests in which there are but few 

 strata of vegetation, during the greater part of the growing 

 season the tall trees constitute almost the entire conspicuous 

 flora. In early spring, however, such forests usually have a 

 low stratum of early -flowering plants (spring beauty, blood- 

 root, dogtooth violet or deer's-tongue, Dutchman's-breeches, 

 etc.), which for a brief period carpet the forest floor. These 



1 In the pine forests of the Tyrol, Wiesner found the illumination in 

 entire shade only one sixtieth to one ninetieth of full sunlight. 



